A method is described for preparing amino-, imino-, and nitrilocarboxylic acids, and their alkali metal salts, starting from alkanolamines. The method employs oxidative dehydrogenation of the alkanolamine(s) in an alkali metal hydroxide medium, using a copper catalyst containing silver.
The subject amino-, imino-, and nitrilocarboxylic acids have the following formula:
R1R2R3Nxe2x80x83xe2x80x83(I)
where R3 is an alkyl group having 1-10 C atoms substituted with a carboxyl group (xe2x80x94COOH),
R1 and R2, which may be the same or different, represent:
hydrogen,
an alkyl group having 1-10 C atoms,
an alkyl group having 1-10 C atoms substituted with a carboxyl group,
a cycloalkyl group having 3-6 C atoms substituted with a carboxyl group,
a cycloalkyl-alkyl group having 3-6 C atoms in the cycloalkyl moiety and 1-10 atoms in the alkyl moiety and substituted with a carboxyl group,
or an alkyl-cycloalkyl group having 1-10 atoms in the alkyl moiety and 3-6 atoms in the cycloalkyl moiety and substituted with a carboxyl group;
wherewith said alkyl groups may be linear or branched,
The described method comprises the following:
(a) Subjecting alkanolamines of formula
R1xe2x80x2R2xe2x80x2R3xe2x80x2Nxe2x80x83xe2x80x83(II)
xe2x80x83wherein the Rxe2x80x2 groups have the same substituents as set forth above for the R groups, except that the aforesaid carboxyl groups (xe2x80x94COOH) are xe2x80x94CH2OH groups, to an oxidative dehydrogenation reaction in aqueous
alkali metal hydroxide medium in the presence of a copper catalyst containing 50-5000 ppm of silver as a promoter;
(b) separating the resulting carboxylate salts from the reaction medium, and optionally purifying said salts or converting them to the corresponding amino acids by precipitation in acid medium;
(c) separating the catalyst from the reaction medium, washing said catalyst with demineralized water, and optionally recycling the catalyst to re-use in step (a), of the same or another production run; and
(d) recovering and collecting the hydrogen liberated in the reaction.
The compounds of formula (I) have been found to be important synthesis intermediaries, e.g. in preparation of N-phosphonomethylglycine (the herbicidal agent known as glyphosate). In our invention, the copper catalyst containing silver as a promoter (e.g. via silver salts), has the advantage that reactivation with each re-use of the catalyst is not required. If a copper catalyst without silver is used, or if a copper catalyst containing a metal or metals other than silver e.g., chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, manganese, zirconium, cobalt, or mixtures of these is used, the catalyst activity fades rapidly with successive reactions.
For the better part of 200 years, it has been known to convert alcohols to alkali metal salts of the corresponding carboxylic acids by heating the alcohols with alkali metal hydroxides (Dumas, 1840, 35 Ann. 129-73).
The reaction has been extended to aminoalcohols; when these are heated in the presence of an alkali metal compound they undergo oxidative dehydrogenation to yield the alkali metal salt of the corresponding amino- or iminoacid; this occurs even without a catalyst (U.S. Pat. No. 2,384,816, preparation of glycine in low yield from diethanolamine and KOH). Known catalysts for use with this reaction include, e.g., cadmium oxide, zinc oxide, palladium, platinum, and activated copper. Hydrogen is liberated. Oxygen or a gas containing oxygen may be introduced to form water from the hydrogen and thereby avoid hazardous accumulations of hydrogen.
Examples from the Patent Literature:
U.S. Pat. No. 2,384,817 (1945), preparation of glycine from monoethanolamine (MEA) and potassium hydroxide at elevated temperature, in an anhydrous medium, with an activated copper catalyst (low yield);
U.S. Pat. No. 3,842,081 (1974), preparation of iminodiacetic acid (IDA) from diethanolamine (DEA) and potassium hydroxide, with a cadmium oxide catalyst (good yield, but cadmium is deemed a toxic substance);
U.S. Pat. No. 3,578,709 (1971), preparation of nitrilotriacetic acid (NTA) from triethanolamine (TEA) and an alkali metal hydroxide, with a zinc oxide catalyst (low yield);
Jap. Pat. 53/7709, preparation of IDA and NTA from DEA and TEA, respectively, in a sodium oxide medium, with a catalyst comprised of Pd or Pt supported on carbon, with injection of oxygen or a gas containing oxygen (low yields, on the order of 70%, and costly precious metal catalysts used to produce a product of relatively low net value);
U.S. Pat. No. 4,782,183, preparation of glycine, IDA, and NTA, from MEA, DEA, and TEA, respectively, and a hydroxide of an [alkaline] alkali metal in aqueous medium, with an activated copper catalyst, at pressures up to 980 kPa (conversion very good), as in the preceding examples, hydrogen liberated in the amount of 2 hydrogen atoms per acetic group, and the further disadvantage pertains that the copper cannot be re-used but must be replaced by fresh copper for each synthesis run, because it becomes depleted (poisoned) in a single use;
U.S. Pat. No. 5,367,112 (1994), preparation of glycine, IDA, and NTA, from MEA, DEA, and TEA, respectively, under the same conditions as in the above-cited patent, but wherewith the activated copper catalyst is promoted with 50-10,000 ppm of an element selected from the group comprising chromium, titanium, niobium, tantalum, tungsten, zirconium, vanadium, molybdenum, manganese, cobalt, nickel, or a mixture of these, the concentration of the catalyst being very high, viz. double that used according to U.S. Pat. No. 4,782,183;
U.S. Pat. No. 5,225,592 (1993), preparation of glycine, IDA, and NTA, from the corresponding alkanolamines and sodium hydroxide, with a copper catalyst, all in aqueous medium and with injection of oxygen or an oxygen-containing gas to avoid emission of hydrogen, the avoidance being achieved viz. by formation of water with the oxygen. Pressure of the system maintained at values up to 11,765 kPa. Drawback again that a new catalyst must be used for each synthesis run;
(PTO Pat. App.) WO 92/06069, regeneration of activated copper used as a catalyst in synthesis of acetic acid derivatives, e.g. preparation of glycine, IDA, and NTA via oxidative dehydrogenation of MEA, DEA, and TEA, respectively. The regeneration is needed because the catalyst loses substantial activity, wherewith in practice absent regeneration it cannot be used more than once and therefore is not economical for industrial applications. In the regeneration, after each synthesis run the copper is treated in an aqueous solution of formaldehyde. A drawback is that effluents resulting from formaldehyde solutions are bactericidal, which creates a disposal problem, not necessary with our invention.
As mentioned above, the above-described copper catalysts, possibly promoted with chromium, molybdenum, titanium, niobium, tantalum, vanadium, zirconium, manganese, tungsten, cobalt, nickel, or with mixtures of these, all experience an appreciable loss of activity with successive uses after the first synthesis run, necessitating re-activation. The loss of activity is attributable to formation of cuprous and cupric oxides on the surface of the copper particles.
It was discovered, in connection with the present invention, that the herein described incorporation of silver in the copper catalyst will greatly increase the catalyst yield as well as substantially increasing the number of synthesis runs for which the catalyst can be used and re-used. Moreover, savings in catalyst used of up to 50 % can be achieved in comparison to the amounts needed according to the above-described prior art patents. The effect is believed to be due to electrochemical protection of the copper by the silver, which reduces or prevents oxidation of the copper. This protective effect, which can be observed by the naked eye as a simple color change if the catalyst is used without a promoter, in comparison to no color change in the promoted catalyst, was clearly established by examination in an electron microscope at magnifications of 150xc3x97 and 5000xc3x97. Particles of un-promoted copper catalyst, and particles of copper catalyst containing silver as a promoter, as described in the present patent application, were subjected to such an examination before and after use, which examination revealed significant differences in the appearance of the un-promoted compared to the promoted activated copper, which stemmed from structural differences in the particles. The micrographs of the copper without the promoter taken before and after use of the catalyst appeared very different: the unused particles had a surface which, while irregular, had a uniform and continuous surface appearance, whereas the used particles showed areas of reduced thickness, with deep fissures and meandering depressions. In contrast, the silver-copper particles had nearly the identical appearance before and after use, indicating a chemical and electrochemical protective effect of the silver on the copper. Consequently, the copper catalyst containing silver had a longer service life than the copper catalyst without silver as a promoter.
The aminoalcohols which may be used in the context of the claimed method are:
MEA, DEA, TEA, N-methylethanolamine,
N-ethylethanolamine, N-ispropylethanolamine, N-butylethanolamine, nonylethanolamine, N-(2-aminoethyl)ethanolamine,
N-(3-aminopropyl)ethanolamine, N,N-dimethylethanolamine,
N,N-diethylethanolamine, N,N-dibutylethanolamine,
N-methyl-N,N-diethanolamine, N-ethyl-N,N-diethanolamine,
N-isopropyl-N,N-diethanolamine, N-butyl-N,N-diethanolamine,
N-ethyl-N-(2-aminoethyl)ethanolamine, and
N-methyl-N-(3-aminopropyl)ethanolamine.
The initial concentration of the alkanolamines may be in the range 15-35 wt % based on the total initial weight of the reaction components, preferably 26-30 wt %. For concentrations above 30 wt %, solubility problems may be encountered, and for concentrations below 26 wt %, correspondingly lower productivity is experienced.
The silver-copper catalyst is first obtained by treating an Alxe2x80x942Cu alloy with sodium hydroxyde, by methods which are per se known. The silver promoter can be incorporated in the alloy; or is applied by precipitation on the catalytic copper by treatment in alkaline medium, from silver salts such as the nitrate, fluoride, chlorate, per chlorate, or lactate, or is provided via simple mixture of activated copper with 50-5000 ppm of finely divided metallic silver.
The amount of catalyst added is in the range 5-25 wt % based on the weight of the alkanolamine to be reacted, preferably 8-12 wt %. At these concentrations, better results are obtained than with un-promoted copper catalyst, or with a copper catalyst promoted with chromium, vanadium, titanium, molybdenum, tungsten, manganese, cobalt, nickel, niobium, tantalum, and zirconium, or mixtures of these.
The solvent is water, to which an alkaline hydroxide has been added in a molar ratio with respect to the alkanolamine which is in the range (stoichiometric) to (stoichiometric +15%); preferably the alkaline hydroxide is added in a stoichiometric amount or in a 5% molar excess. The hydroxide may be that of any alkali metal (e.g. lithium, sodium, or potassium), provided that the salts of said hydroxide with the synthesized amino acid are soluble in the reaction medium at the synthesis temperature and pressure.
The reaction is carried out at 100-220xc2x0 C., preferably 120-190xc2x0 C., at a pressure in the range 490-1470 kPa, preferably 784-1170 kPa, particularly preferably 883-980 kPa.
The conversion of the alkanolamine to the corresponding amino acid occurs with liberation of hydrogen. The hydrogen may be compressed and stored for use in other processes.